Integrated circuits ("ICs") are well known. ICs include literally hundreds of thousands, even millions, of transistors which are logically coupled to one another to provide arithmetic and logic units, address decoders, memory, control registers, and the like. While the computational ability and logical processing provided by such ICs can be quite extraordinary, the capabilities of these ICs are of little value unless the IC is coupled to receive inputs from an external device or provide outputs to an external device. The electrical connection between the internal transistors provided on a semiconductor substrate inside an IC and the circuit traces provided on a printed circuit card or the like is usually achieved through a conductive pin mounted to the housing for the IC. One end of the conductive pin extends outwardly from the housing so it may be soldered or otherwise connected to a circuit trace on a printed circuit card. The other end of the conductive pin extends inside the housing and is coupled to an I/O pad on the substrate through conductive elements, usually gold wires, which are bonded at one end to the pin and at the other end to the I/O pad.
In order to provide a signal to an external device which is very susceptible to electrical noise on the printed circuit card, the current supply in the signals provided to the I/O pads have substantially more current than those signals provided internally between transistors of the IC. This additional current is provided by drivers at the I/O pad responding to signals on the signal bus or buses coupled to an I/O pad. Because the I/O pads are coupled to the signal buses on the semiconductor substrate, signals on these internal buses cause the drivers to operate. For example, the internal data bus of an IC has many data words communicated across it, only some of which are transferred to an external device. However, all of this data operates the drivers at the I/O pads. Because most state of the art ICs communicate signals at very high switching rates, noise may be induced in conductive elements in the vicinity of the I/O pads being driven by the data on the internal data bus, for example. This electrical noise may affect the operation of an external device coupled to the I/O pads. For example, analog/digital and digital/analog converters may generate or use precision data or signals. Such circuits, for example, require a reference voltage having little variation about its reference value so the component correctly generates digital data corresponding to an analog signal or accurately generates an analog signal corresponding to digital data. Such a precision electronic component may be coupled to an IC through the I/O pads. Signals switching over an internal data bus coupled to I/O pads may induce noise which affects the reference voltages or other signals required for operation of the external device.
In an effort to address electrical noise problems in ICs, a number of noise reducing circuits have been devised. For example, some have determined that the power and ground conductive pins of an IC respond like an inductor when current loads change through the pins. Consequently, one way to reduce switching noise is to reduce power and ground pin inductance by adding additional power and ground pins to the IC. However, by allocating more of the conductive pins for coupling the semiconductor substrate of the IC to external power and ground sources, fewer pins are available for actual data or control signals. Another solution to the problem of switching noise is to reduce the rate of change of current through power and ground pin inductances. For example, U.S. Pat. No. 4,825,102 describes an output driver which turns one driving circuit off before it turns another driving circuit on in an effort to reduce the changing current loads. Other methods, such as that taught in U.S. Pat. No. 4,827,159, include using inverters with different switching thresholds to cause drivers to turn on and off at different times, again with the intention of reducing changing current loads. Another approach, described in U.S. Pat. No. 5,426,376, provides a transient switching circuit and a logic holding circuit which separate first and second power and ground voltage sources from one another to reduce the switching noise. While each of these approaches reduces the amount of current, and consequently noise, it usually requires that most, if not all, of the I/O pad drivers for the IC implement the noise reducing circuitry. The pervasive implementation of these solutions add complexity and cost to the fabrication of IC's.
What is needed is a way to electrically isolate external electronic components or devices or electrical noise generated by changing currents at the I/O pads without requiring modification of most of the I/O pads of an IC.